eye 
GILL CHARACTERS 19 
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by contrasting the highly modified gill apparatus of a bony 
fish with the more archaic type of the shark. 
In the sharks (p. 73) the gill slits pierce separately 
the throat wall, as in the lamprey, and thus retain their 
primitive segmental arrangement (Fig. 11). Their number 
is usually five on either side, but in an archaic form (Hep- 
tanchus, p. 88) may be increased to seven. Above and 
in front of the line of gill slits occurs a small opening 
leading into the gullet, the spzvacle (SP). This, though 
at present possessing but few gill lamella, and therefore 
ee SS ett 
of little respiratory value, was doubtless quite like its 
| neighbours before its gill-supporting tissue became of value 
__~ in suspending the lower jaw. It may now aid the mouth 
opening in admitting water to the gills. At the left of 
; the figure (Fig. 11), the narrow slit-like openings of the 
} gill clefts are seen at GC: at the right, where the upper 
portion of the head has been removed, the gill lamellz 
are shown at GF; the tissue intervening between the 
gill pouches is reduced to a thin tendinous septum, S7, 
at whose inner rim is the cartilaginous gill arch or bar, 
GB, supporting the branchial vessels, GV. 
In the gill region of a bony fish (Fig. 12) a number of 
modified characters are now evident: the spiracle has 
become obliterated; the number of gill bars reduced — 
in one form but two on either side remaining. These 
have become closely pressed together, and bent backward, 
receding from the surface of the head: their gill lamellz 
have become larger and more numerous, their intervening 
septum, SZ, reduced in size. The gills no longer open| 
separately at the surface, but into an outer branchial 
chamber formed and protected by a large overlapping 
scale, or opercle, OP. This shield-like organ is hinged 
at its anterior margin and opens or shuts rhythmically as 
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